Magnetic-drive centrifugal pumps may be used to pump fluids, such as caustic and hazardous liquids. Instead of shaft seals, a magnetic-drive pump features a pump shaft separated from a drive shaft by a containment shell. The drive shaft is arranged to rotate with one magnetic assembly, which is magnetically coupled to another magnetic assembly. The magnetic assemblies cooperate to apply torque to the pump shaft to pump a fluid contained by the containment shell.
In a magnetic-drive centrifugal pump, the impeller is exposed to the pumped fluid. The magnetic assembly of the impeller may be encapsulated directly with a polymeric layer to protect the magnetic assembly from oxidation or corrosion by the pumped fluid. However, the polymeric layer is generally semi-permeable or sufficiently permeable to allow some of the pumped fluid (or constituents) to migrate through the polymeric layer to the magnetic assembly. Over time, one or more magnets of the magnetic assembly may be oxidized or corroded from exposure to the pumped fluid. When rust or other deposits form on a magnet, the properties of the magnet may change which may degrade performance of the pump in any of the following ways: (1) delamination of the polymeric layer from the magnet, (2) increased size of the magnet along with decreased axial clearance between the impeller and the pump interior, and (3) reduction in the magnitude of the magnetic field produced by the magnets. If a decrease in axial clearance is great enough, rubbing contact between the impeller and the pump interior may lead to failure of the pump. For example, the integrity of the containment shell may be compromised by mechanical scraping of the impeller or the pumped fluid may be contaminated by chemical interaction with an exposed portion of the magnetic assembly. If the magnetic coupling force is reduced by degradation of the impeller magnets, the pump may operate with reduced torque and lower pumping capacity. Thus, a need exists for improving the protection of the magnetic assembly of the impeller from the pumped fluid.